Geomagnetic Pulsations!

Every now and then geomagnetic pulsations are observed wherever one is measuring Earth’s magnetic field at high latitudes. Such pulsations are categorized according to their period in classes PC1 – PC5. PC stands for “pulsation continue”, which is French.

This morning we observe pulsations in all the TGO magnetometers located in the auroral zone, for example in Tromsø as shown below:

PC5 pulsations seen from Tromsø magnetic observatory

The pulsations seen above with a period of about 5 minutes are within the class of PC5, which is defined as oscillations with periods between 150 and 600 seconds.

PC4 and 5 pulsations are attributed to Ultra Low Frequency waves in the magnetosphere. These ULF waves are magnetohydrodynamic plasma waves propagating throughout the magnetosphere, and are a resonance phenomenon caused by the interaction between the solar wind and the magnetosphere. A satellite located in the magnetosphere measuring the electric or magnetic field, would see oscillations with the same period.

As can be seen in the plot (blue curve), the amplitude of these oscillations are of the order of 50 nT, so these cannot be considered large perturbations compared to other phenomena such as substorms. However, from a space weather perspective, and especially for those who are worried about ground induced currents, these might cause challenges. Below we show the last 24 hours of the differentiated magnetic field (dB/dt) in Tromsø. dB/dt is a good proxy of the induction in the ground and thus the possibility of induced currents in the power grid.

dB/dt for the last 24 hours in the minute of writing.

In the interval between UT 3 and 9, the pulsations are creating a clear signal in the plot. However, the substorm at about 22 UT produced a much larger signal and, therefore, we can conclude that these pulsations didn’t pose an out-of-the-ordinary challenge for power grid operators. The dB/dt signal is often seen to peak during substorms, on an ordinary day values of 4-5 nT/s are reached. During geomagnetic storms values above 10 nT/s covering large geographical areas are often seen. The dB/dt, both real-time and historical plots can be investigated here.